diff --git a/src/Type/Constant/ConstantArrayType.php b/src/Type/Constant/ConstantArrayType.php
index 6cff7ef3af..0593049044 100644
--- a/src/Type/Constant/ConstantArrayType.php
+++ b/src/Type/Constant/ConstantArrayType.php
@@ -47,6 +47,7 @@
 use PHPStan\Type\Traits\UndecidedComparisonTypeTrait;
 use PHPStan\Type\Type;
 use PHPStan\Type\TypeCombinator;
+use PHPStan\Type\TypeUtils;
 use PHPStan\Type\UnionType;
 use PHPStan\Type\VerbosityLevel;
 use function array_keys;
@@ -699,6 +700,57 @@ public function getOffsetValueType(Type $offsetType): Type
 
 	public function setOffsetValueType(?Type $offsetType, Type $valueType, bool $unionValues = true): Type
 	{
+		if ($offsetType !== null) {
+			$scalarKeyTypes = $offsetType->toArrayKey()->getConstantStrings();
+			if (count($scalarKeyTypes) === 0) {
+				$integerRanges = TypeUtils::getIntegerRanges($offsetType);
+				if (count($integerRanges) > 0) {
+					foreach ($integerRanges as $integerRange) {
+						$finiteTypes = $integerRange->getFiniteTypes();
+						if (count($finiteTypes) === 0) {
+							break;
+						}
+
+						foreach ($finiteTypes as $finiteType) {
+							$scalarKeyTypes[] = $finiteType;
+						}
+					}
+				}
+			}
+
+			// turn into tagged union for more precise results
+			if (
+				count($scalarKeyTypes) >= 2
+				&& count($scalarKeyTypes) < ConstantArrayTypeBuilder::ARRAY_COUNT_LIMIT
+			) {
+				$hasNewKey = false;
+				foreach ($scalarKeyTypes as $scalarKeyType) {
+					$existingKeyFound = false;
+					foreach ($this->keyTypes as $existingKeyType) {
+						if ($existingKeyType->getValue() === $scalarKeyType->getValue()) {
+							$existingKeyFound = true;
+							break;
+						}
+					}
+					if (!$existingKeyFound) {
+						$hasNewKey = true;
+						break;
+					}
+				}
+
+				if ($hasNewKey) {
+					$arrayTypes = [];
+					foreach ($scalarKeyTypes as $scalarKeyType) {
+						$builder = ConstantArrayTypeBuilder::createFromConstantArray($this);
+						$builder->setOffsetValueType($scalarKeyType, $valueType);
+						$arrayTypes[] = $builder->getArray();
+					}
+
+					return TypeCombinator::union(...$arrayTypes);
+				}
+			}
+		}
+
 		$builder = ConstantArrayTypeBuilder::createFromConstantArray($this);
 		$builder->setOffsetValueType($offsetType, $valueType);
 
diff --git a/tests/PHPStan/Analyser/nsrt/bug-9907.php b/tests/PHPStan/Analyser/nsrt/bug-9907.php
new file mode 100644
index 0000000000..0985776490
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-9907.php
@@ -0,0 +1,20 @@
+<?php declare(strict_types = 1);
+
+namespace Bug9907;
+
+use function PHPStan\Testing\assertType;
+
+class HelloWorld
+{
+	/**
+	 * @param 'foo'|'bar' $key
+	 */
+	public function sayHello(string $key): void
+	{
+		$a = [];
+		$a['id'] = null;
+		$a[$key] = 'string';
+
+		assertType("array{id: null, bar: 'string'}|array{id: null, foo: 'string'}", $a);
+	}
+}
diff --git a/tests/PHPStan/Analyser/nsrt/constant-array-type-set.php b/tests/PHPStan/Analyser/nsrt/constant-array-type-set.php
index 9ae0b88828..0f32789b28 100644
--- a/tests/PHPStan/Analyser/nsrt/constant-array-type-set.php
+++ b/tests/PHPStan/Analyser/nsrt/constant-array-type-set.php
@@ -94,7 +94,7 @@ public function doBar5(int $offset): void
 	{
 		$a = [false, false, false];
 		$a[$offset] = true;
-		assertType('non-empty-array<int<0, 4>, bool>', $a);
+		assertType("array{0: false, 1: false, 2: false, 4: true}|array{false, false, false, true}|array{false, false, true}|array{false, true, false}|array{true, false, false}", $a);
 	}
 
 	public function doBar6(bool $offset): void
diff --git a/tests/PHPStan/Analyser/nsrt/set-constant-union-offset-on-constant-array.php b/tests/PHPStan/Analyser/nsrt/set-constant-union-offset-on-constant-array.php
new file mode 100644
index 0000000000..66dace8c80
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/set-constant-union-offset-on-constant-array.php
@@ -0,0 +1,70 @@
+<?php
+
+namespace SetConstantUnionOffsetOnConstantArray;
+
+use function PHPStan\Testing\assertType;
+
+class Foo
+{
+
+	/**
+	 * @param array{foo: int}  $a
+	 */
+	public function doFoo(array $a): void
+	{
+		$k = rand(0, 1) ? 'a' : 'b';
+		$a[$k] = 256;
+		assertType('array{foo: int, a: 256}|array{foo: int, b: 256}', $a);
+	}
+
+	/**
+	 * @param array{foo: int}  $a
+	 * @param int<1,5> $intRange
+	 */
+	public function doBar(array $a, $intRange): void
+	{
+		$a[$intRange] = 256;
+		assertType('array{foo: int, 1: 256}|array{foo: int, 2: 256}|array{foo: int, 3: 256}|array{foo: int, 4: 256}|array{foo: int, 5: 256}', $a);
+	}
+
+	/**
+	 * @param array{foo: int}  $a
+	 * @param int<0, max> $intRange
+	 */
+	public function doInfiniteRange(array $a, $intRange): void
+	{
+		$a[$intRange] = 256;
+		assertType('non-empty-array<\'foo\'|int<0, max>, int>', $a);
+	}
+
+	/**
+	 * @param array{foo: int}  $a
+	 * @param int<0, 5>|int<10, 15> $intRange
+	 */
+	public function doUnionOfRanges(array $a, $intRange): void
+	{
+		$a[$intRange] = 256;
+		assertType('array{foo: int, 0: 256}|array{foo: int, 10: 256}|array{foo: int, 11: 256}|array{foo: int, 12: 256}|array{foo: int, 13: 256}|array{foo: int, 14: 256}|array{foo: int, 15: 256}|array{foo: int, 1: 256}|array{foo: int, 2: 256}|array{foo: int, 3: 256}|array{foo: int, 4: 256}|array{foo: int, 5: 256}', $a);
+	}
+
+	/**
+	 * @param array{foo: int}  $a
+	 * @param int<0, 3>|int<2, 4> $intRange
+	 */
+	public function doOverlappingRanges(array $a, $intRange): void
+	{
+		$a[$intRange] = 256;
+		assertType('array{foo: int, 0: 256}|array{foo: int, 1: 256}|array{foo: int, 2: 256}|array{foo: int, 3: 256}|array{foo: int, 4: 256}', $a);
+	}
+
+	/**
+	 * @param array{0: 'a', 1: 'b'}  $a
+	 * @param int<0,1> $intRange
+	 */
+	public function doExistingKeys(array $a, $intRange): void
+	{
+		$a[$intRange] = 'c';
+		assertType("array{'a'|'c', 'b'|'c'}", $a);
+	}
+
+}
diff --git a/tests/PHPStan/Rules/Methods/CallMethodsRuleTest.php b/tests/PHPStan/Rules/Methods/CallMethodsRuleTest.php
index 0ab14fd52a..2bb2c5c2e6 100644
--- a/tests/PHPStan/Rules/Methods/CallMethodsRuleTest.php
+++ b/tests/PHPStan/Rules/Methods/CallMethodsRuleTest.php
@@ -3853,4 +3853,12 @@ public function testBug13805(): void
 		$this->analyse([__DIR__ . '/data/bug-13805.php'], []);
 	}
 
+	public function testBug7978(): void
+	{
+		$this->checkThisOnly = false;
+		$this->checkNullables = true;
+		$this->checkUnionTypes = true;
+		$this->analyse([__DIR__ . '/data/bug-7978.php'], []);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Methods/ReturnTypeRuleTest.php b/tests/PHPStan/Rules/Methods/ReturnTypeRuleTest.php
index 84e81f0f33..9b3bb88db0 100644
--- a/tests/PHPStan/Rules/Methods/ReturnTypeRuleTest.php
+++ b/tests/PHPStan/Rules/Methods/ReturnTypeRuleTest.php
@@ -1310,4 +1310,9 @@ public function testBug9669(): void
 		$this->analyse([__DIR__ . '/data/bug-9669.php'], []);
 	}
 
+	public function testBug9907(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-9907.php'], []);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Methods/data/bug-7978.php b/tests/PHPStan/Rules/Methods/data/bug-7978.php
new file mode 100644
index 0000000000..4b3f951835
--- /dev/null
+++ b/tests/PHPStan/Rules/Methods/data/bug-7978.php
@@ -0,0 +1,29 @@
+<?php declare(strict_types = 1);
+
+namespace Bug7978;
+
+class Test {
+
+	const FIELD_SETS = [
+		'basic'   => ['username', 'password'],
+		'headers' => ['app_id', 'app_key'],
+	];
+
+	/**
+	 * @param array<string, string> $credentials
+	 */
+	public function acceptCredentials(array $credentials): void
+	{
+	}
+
+	public function doSomething(): void
+	{
+		foreach (self::FIELD_SETS as $type => $fields) {
+			$credentials = [];
+			foreach ($fields as $field) {
+				$credentials[$field] = 'fake';
+			}
+			$this->acceptCredentials($credentials);
+		}
+	}
+}
diff --git a/tests/PHPStan/Rules/Methods/data/bug-9907.php b/tests/PHPStan/Rules/Methods/data/bug-9907.php
new file mode 100644
index 0000000000..39086dbd70
--- /dev/null
+++ b/tests/PHPStan/Rules/Methods/data/bug-9907.php
@@ -0,0 +1,34 @@
+<?php declare(strict_types = 1);
+
+namespace Bug9907Rule;
+
+class Demo
+{
+	/**
+	 * @phpstan-param array{street: string, city: string} $address1
+	 * @phpstan-param array{street: string, city: string} $address2
+	 *
+	 * @phpstan-return array{
+	 *     street?: array{change_to: string},
+	 *     city?: array{change_to: string},
+	 *     variation_count?: int<1, max>
+	 * }
+	 */
+	public function diffAddresses(array $address1, array $address2): array
+	{
+		$addressDifference = array_diff_assoc($address1, $address2);
+		$differenceDetails = [];
+
+		foreach ($addressDifference as $name => $differenceValue) {
+			$differenceDetails[$name] = [
+				'change_to' => $differenceValue,
+			];
+		}
+
+		if (!empty(count($differenceDetails))) {
+			$differenceDetails['variation_count'] = count($differenceDetails);
+		}
+
+		return $differenceDetails;
+	}
+}